A web browser may comprise functionality that is configured to access and request resources embedded within a webpage to be rendered by the web browser. However, a content modifier such as a third party add-on of the web browser may be configured to modify, substitute or block one or more particular types of resources associated with the webpage prior to rendering by the web browser. Such content modification functionality may typically utilize a range of techniques to modify the webpage. These techniques include (i) adding content to the webpage prior to rendering by the web browser; (ii) removing content from the webpage prior to rendering by the web browser; (iii) blocking retrieval of content by the web browser prior to rendering by the web browser; (iv) substituting content associated with the webpage prior to rendering by the web browser; and (v) modifying one or more display attributes associated with content prior to rendering by the web browser. Typically, these modifications are performed automatically as a background process and without the knowledge or explicit consent of a user of the web browser or a publisher of the web content. Moreover, these modifications may negatively impact the functionality and aesthetics of the content, thereby compromising the integrity of the webpage.